Grease and 9-inch square pan, then line with non-stick baking paper, make sure to cut excess so that it has flaps on the side. Spray again with cooking spray.
In a mixer bowl, bloom gelatin powder in ½ cup cold water. Set aside.
In a saucepan, pour in granulated sugar, light corn syrup and ¼ cup water. Place over medium low heat.
Heat the sugar until it reaches 240F/115Once the sugar syrup almost reaches the correct temperature, start to whip the bloomed gelatin, on high speed, slowly and gradually pour the sugar syrup.
Continue whipping on high speed until the mixture fluffs. About 10-12 minutes.
Add in the salt and vanilla extract. Continue whipping until salt and vanilla is mixed in.
Pour mixture into prepared pan. Flatten with a greased offset spatula. Dust the top with the powder mixture. Place in the fridge to set.
Dust a chopping board with more cornstarch-powdered sugar, tip the set marshmallow into the board, cut using a greased sharp edge knife or greased cookie cutters, as desired. Dust with more powder then sift to remove excess.